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stone in central Italy because they were looking for evidence that the Italian
peninsula had "rotated" in the geological past during tectonic intetactions
with Western Europe. Laboratory analyses of these cores, however, showed
that there was so much movement of various limestone layers, one upon an-
other, that no detailed history of plate motion could be obtained. What
did
emerge was an unexpected and magnificent record of magnetic reversals. Be-
cause these Italian limestones contained a rich record of
microfossib
(mainly
the skeletons of single-celled organisms called planktonic fotaminiferans),
the individual reversals discovered by Lowrie and Alvarez could be correlated
both with other land-based sections and with deep sea lava, where reversal
boundaries were dated using radiometric techniques. Fot the first time, Euro-
pean fossils could be associated with radiometric dates by using the magnetic
reversal history as a go-between, and for the first time it was confirmed that
the detailed and continuous deep sea record of change in magnetic field po-
larity could also be detected in sedimentary rocks found on land.
The magnetic reversal record discovered in the thick, white limestone
of Italy contained several surprises. First and foremost was the discovery that
for mote than half of the 60-million-year Cretaceous Period, there
were no
magnetic reversals at all.
Before this, it had been assumed that the cadence of
magnetic pole reversal was rather constant, with a shift every half-million to
million years or so, and indeed much of the geological record reveals just this
type of pattern. But for reasons still unclear, the mechanism that
creates
mag-
netic field teversals occasionally went on holiday. From about 118 to about
83 million years ago, the magnetic field was locked into normal polarity.
Magnetostratigraphic dating is useless for this interval. However, the first re-
versal
after
this long interval of normal polarity is one of the most recogniz-
able in the entire geological column. This "long normal," as it came to be
called, was followed by a return to reversed and then the restoration of nor-
ma! polarity in shorter intervals, which produced a readily recognizable "fin-
gerprint" of polarity. Anywhere on earth where you know (from fossils or ra-
diometric dating) that you are in the Cretaceous, and where you have just
received a reversed-polarity signal following a long interval of normal polarity,
